Yttrium-based sprayed coating and making method

ABSTRACT

An yttrium-base sprayed coating is obtained by thermally spraying yttrium oxide, yttrium fluoride or yttrium oxyfluoride onto a substrate to form a coating of 10-500 μm thick, and chemically cleaning the coating with a cleaning liquid of organic acid, inorganic acid or a mixture thereof until the population of particles with a size of up to 300 nm becomes no more than 5 particles/mm 2  of the coating surface. The yttrium-base sprayed coating exhibits high corrosion resistance even in a halogen gas plasma atmosphere and prevents yttrium-base particles from spalling off during etching treatment.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 15/220,652filed on Jul. 27, 2016, which is based upon and claims the benefit ofpriority of the non-provisional application which claims priority under35 U.S.C. §119(a) on Patent Application No. 2015-151568 filed in Japanon Jul. 31, 2015, the entire contents of which are incorporated hereinby reference.

TECHNICAL FIELD

This invention relates to an yttrium-base sprayed coating formed bythermally spraying yttrium oxide, yttrium fluoride and/or yttriumoxyfluoride, which is suited as a low dusting coating on parts andarticles used in a corrosive plasma atmosphere as encountered in asemiconductor device fabrication process.

BACKGROUND ART

In the prior art process for manufacturing semiconductor devices,treatment is often carried out in a corrosive halogen-base gas plasmaatmosphere. Typical corrosive halogen-base gases are fluorine-base gasessuch as SF₆, CF₄, CHF₃, ClF₃ and HF and chlorine-base gases such as Cl₂,BCl₃ and HCl. The equipment used for such treatment typically includesparts or components having corrosion resistant coatings on theirsurface. For example, parts or components having coatings formed byspraying yttrium oxide (Patent Document 1) and yttrium fluoride (PatentDocuments 2 and 3) to the surface of metallic aluminum and aluminumoxide ceramic substrates are known to be fully corrosion resistant andused in practice.

As the current semiconductor technology aims at higher integration, thesize of interconnections is approaching to 20 nm or less. In the devicefabrication process, yttrium-base particles may spall off the surface ofyttrium-base coatings on the parts during etching treatment and fallonto silicon wafers to interfere with the etching treatment. This causesto reduce the manufacture yield of semiconductor devices. There is atendency that the number of yttrium-base particles spalling off theyttrium-base coating surface is high at the early stage of etchingtreatment and decreases with the lapse of etching time. Patent Documents4 and 5 relating to the spraying technology are also incorporated hereinby reference.

CITATION LIST

Patent Document 1: JP 4006596 (U.S. Pat. No. 6,852,433)

Patent Document 2: JP 3523222 (U.S. Pat. No. 6,685,991)

Patent Document 3: JP-A 2011-514933 (US 20090214825)

Patent Document 4: JP-A 2008-133528 (U.S. Pat. No. 8,349,450)

Patent Document 5: JP 4591722 (US 20130122218)

DISCLOSURE OF INVENTION

An object of the invention is to provide an yttrium-base sprayed coatingwhich is formed by thermally spraying one or more compounds selectedfrom among yttrium oxide, yttrium fluoride, and yttrium oxyfluoride,capable of substantially preventing yttrium-base particles from spallingoff the coating surface during etching or similar treatment, and thussuited for use as a low dusting coating on parts or articles used in acorrosive plasma atmosphere during the semiconductor device fabricationprocess.

When a coating of yttrium oxide, yttrium fluoride or yttrium oxyfluorideis formed by plasma spraying, a particulate material is melted in aplasma flame into droplets, after which droplets deposit and solidify ona substrate to form a coating. If the size of material particles is toosmall, some particles may not enter the flame, but deposit on thecoating in the unmelted state. Also, once particles are melted,sometimes droplets may burst on the coating into finer droplets, whichwill deposit on the coating as finer particles. Such fine (unmelted orburst) particles which deposit on the coating surface in the course ofspraying are covered with the following droplets and thus integratedinto a dense coating, whereas foreign particles which deposit on thecoating surface near the end of spraying remain bonded as such. It isdifficult to remove the bonded particles by ultrapure water cleaning,ultrasonic cleaning or the like. If fine particles spall off duringetching treatment, they become a source of dusting. Patent Document 5proposes physical removal of sticky particles (i.e., particles which arenot removable by ultrapure water cleaning or ultrasonic cleaning) bypolishing or blasting. However, physical removal such as polishing isnot so effective because the treatment itself generates fine particles.

The inventors have found that an improved yttrium-base sprayed coatingis obtained by thermally spraying one or more compounds selected fromamong yttrium oxide, yttrium fluoride, and yttrium oxyfluoride to form acoating of 10 to 500 μm thick, and chemically cleaning the coating witha cleaning liquid in the form of an aqueous solution of organic acid orinorganic acid or a mixture thereof for effectively removingyttrium-base particles anchored to the coating surface until thepopulation of particles having a size of up to 300 nm becomes no morethan 5 particles/mm² of the coating surface. Since the resultingyttrium-base sprayed coating prevents yttrium-base particles fromspalling off to cause a failure during subsequent etching treatment, itis suitable for use as a low dusting coating on parts and articles usedin a corrosive plasma atmosphere in the semiconductor device fabricationprocess.

In one aspect, the invention provides an yttrium-base sprayed coatingcomprising one or more compounds selected from the group consisting ofyttrium oxide, yttrium fluoride, and yttrium oxyfluoride and having athickness of 10 to 500 μm, wherein particles with a size of up to 300 nmare present on a coating surface in a population of no more than 5particles per square millimeters.

Preferably the yttrium-base sprayed coating has a thickness of 80 to 400μm.

Typically the yttrium-base sprayed coating is sprayed onto a surface ofa substrate of metallic aluminum, aluminum oxide or metallic silicon.

In another aspect, the invention provides a method for preparing ayttrium-base sprayed coating, comprising the steps of thermally sprayinga particulate spray material comprising at least one compound selectedfrom the group consisting of yttrium oxide, yttrium fluoride, andyttrium oxyfluoride to form a yttrium-base sprayed coating having athickness of 10 to 500 μm and chemically cleaning a surface of thecoating with a cleaning liquid which is an organic acid aqueoussolution, inorganic acid aqueous solution or organic acid/inorganic acidaqueous solution until a population of particles with a size of up to300 nm is no more than 5 particles per square millimeters of the coatingsurface.

The cleaning liquid is preferably an aqueous solution of an acidselected from the group consisting of a monofunctional carboxylic acid,difunctional carboxylic acid, trifunctional carboxylic acid, hydroxyacid, sulfonic acid, nitric acid, sulfuric acid, carbonic acid,hydrofluoric acid, and acidic ammonium fluoride or a mixture thereof.Typically, the monofunctional carboxylic acid is formic acid or aceticacid, the difunctional carboxylic acid is maleic acid, tartaric acid orphthalic acid, the trifunctional carboxylic acid is citric acid, thehydroxy acid is lactic acid, and the sulfonic acid is methanesulfonicacid.

In a preferred embodiment, the chemical cleaning step includes immersingthe yttrium-base sprayed coating in the cleaning liquid to dissolve thecoating to a depth of at least 0.01 μm from its surface for therebyremoving particles with a size of up to 300 nm on the coating surface.

ADVANTAGEOUS EFFECTS OF INVENTION

The yttrium-base sprayed coating of the invention exhibits highcorrosion resistance during treatment in a corrosive halogen-base gasplasma atmosphere, and prevents dusting as a result of yttrium-baseparticles spalling off during etching or similar treatment in thesemiconductor device fabrication process, which is effective forimproving the fabrication yield of semiconductor devices. Theyttrium-base sprayed coating is thus suitable for use as a low dustingcoating on parts and articles which are exposed to a corrosive plasmaatmosphere.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1, 2, 3 and 4 are SEM images of the surface of yttrium-basesprayed coatings in Examples 1, 2, 3 and 4, respectively.

FIGS. 5 and 6 are SEM images of the surface of yttrium-base sprayedcoatings in Comparative Examples 1 and 2, respectively.

DESCRIPTION OF PREFERRED EMBODIMENTS

The yttrium-base sprayed coating of the invention is formed by thermallyspraying one or more compounds selected from among yttrium oxide,yttrium fluoride, and yttrium oxyfluoride.

Thermal spraying to a substrate is desirably atmospheric plasma sprayingor vacuum plasma spraying. The plasma gas used herein may benitrogen/hydrogen, argon/hydrogen, argon/helium, argon/nitrogen, argonalone, or nitrogen gas alone, but not limited thereto. Examples of thesubstrate subject to thermal spraying include, but are not limited to,substrates of stainless steel, aluminum, nickel, chromium, zinc, andalloys thereof, metal silicon, aluminum oxide, aluminum nitride, siliconnitride, silicon carbide, and quartz glass when parts or components ofthe semiconductor fabrication equipment are contemplated. The conditionsunder which yttrium oxide, yttrium fluoride or yttrium oxyfluoride isthermally sprayed are not particularly limited. The thermal sprayingconditions may be determined as appropriate depending on the identity ofsubstrate, the particle size and composition of spray material, and aparticular application of the resulting sprayed component.

For example, when an yttrium oxide coating is formed on a metal aluminumsubstrate, it may be deposited by argon/hydrogen atmospheric plasmaspraying using yttrium oxide powder having an average particle size D50of about 20 μm and a gas mixture of 40 L/min of argon and 5 L/min ofhydrogen. The thermal spraying conditions including a spray distance,current value and voltage value may be determined as appropriatedepending on a particular application of the sprayed component.Likewise, the feed rates of argon and hydrogen gases may be suitablyadjusted.

The sprayed coating, i.e., yttrium-base sprayed coating should have athickness of 10 to 500 μm. A coating of less than 10 μm thick may beless corrosion resistant or allow the substrate surface to be partlyexposed in the cleaning step to be described below. A coating of morethan 500 μm thick may simply add to the cost because no furtherimprovement in corrosion resistance is expectable. The thickness of thecoating is preferably 80 to 400 μm, more preferably 100 to 400 μm, andeven more preferably 100 to 300 μm.

According to the invention, the surface of the yttrium-base sprayedcoating is then cleaned with a preselected cleaning liquid to removeyttrium-base particles anchored thereto until the population (or number)of yttrium-base particles with a size of up to 300 nm becomes no morethan 5 particles/square millimeters (mm²) of the coating surface. It is,of course, most preferred that the population of yttrium-base particleswith a size of up to 300 nm on the coating surface be 0. As long as thepopulation is no more than 5 particles/mm², dusting to such an extent asto invite a substantial loss of production yield does not occur duringetching treatment in the semiconductor device fabrication process. Asused herein, the “size” of yttrium-base particles refers to the maximumdiameter of individual particles measured by microscopy under a scanningelectron microscope (SEM) or the like. As seen from the images of FIGS.5 and 6, no or only a few particles with a size in excess of 300 nm arepresent on the sprayed coating surface. Removal of particles with a sizeof up to 300 nm means removal of substantially all inhibitory particles.

The cleaning liquid is an aqueous solution of organic acid, aqueoussolution of inorganic acid or aqueous solution of mixed organic andinorganic acids. The organic acid is not particularly limited as long asit is water-soluble. Suitable organic acids include, but are not limitedto, monofunctional carboxylic acids such as formic acid and acetic acid,difunctional carboxylic acids such as maleic acid, tartaric acid andphthalic acid, trifunctional carboxylic acids such as citric acid,hydroxy acids such as lactic acid, and sulfonic acids such asmethanesulfonic acid. Inter alia, tartaric acid and citric acid arepreferred because they are edible, nontoxic and easy to handle. Theinorganic acid is not particularly limited as long as it iswater-soluble. Suitable inorganic acids include nitric acid, sulfuricacid, carbonic acid, hydrofluoric acid, and acidic ammonium fluoride.

The cleaning technique is not particularly limited. Preferably, a partor component in the form of a substrate having the yttrium-base sprayedcoating formed on its surface is wholly immersed in the cleaning liquidbecause this technique is effective and efficient. For those substratesof metallic aluminum and silicon which are readily dissolved in acid,the area of the substrate that should avoid corrosion with acid isdesirably masked with resin tape or sheet when a strong acid is used forcleaning. Cleaning without masking is possible when a weak organic acidis used for cleaning, for example, a carboxylic acid or hydroxy acidsuch as phthalic acid, tartaric acid or citric acid. For thosesubstrates of quart glass or Al₂O₃ ceramics which are acid resistant,cleaning without masking is possible even with a strong acid solutionsuch as nitric acid. In some cases, a buffer solution based on acombination of acid and salt may be used as the cleaning liquid.

The yttrium-base sprayed coating is chemically cleaned with the cleaningliquid to dissolve a thin layer from the coating surface for removingparticles with a size of up to 300 nm which become a source of dusting.The dissolution depth is preferably at least 0.01 μm from the originalcoating surface. Although the upper limit of dissolution depth is notcritical, the dissolution depth is preferably up to 20 μm. Morepreferably the dissolution depth is 1 to 20 μm from the coating surface.A dissolution depth of less than 0.01 μm may be insufficient to removeparticles with a size of up to 300 nm and fail to reach a population ofno more than 5 particles/mm². A dissolution depth in excess of 20 μm maysimply make the coating thinner without further improvements in particleremoval.

After cleaning, the coating is rinsed with ultrapure water to thoroughlyremove the acid and dried in vacuum or under atmospheric pressure.

When a secondary electron image (magnification ×10,000 or more) of thedry coating surface is observed under SEM, yttrium-base particles havinga size of up to 300 nm on the coating surface are detectable. Accordingto the invention, yttrium-base particles are removed from the coatingsurface by the cleaning step until the population of particles reachesno more than 5 particles/mm² of the surface.

EXAMPLE

Examples are given below by way of illustration and not by way oflimitation.

Examples 1 to 4 and Comparative Examples 1 and 2 Preparation of SprayedCoating

An yttrium-base sprayed coating was obtained by thermally spraying thecoating material shown in Table 1 onto a surface of a substrate of thematerial shown in Table 1, immersing the coated substrate in a cleaningliquid, which was an aqueous solution of the cleaning agent shown inTable 1, to clean the coating surface, thoroughly rinsing with ultrapurewater, and vacuum drying. The surface of the yttrium-base coating thusobtained was observed under SEM, and yttrium-base particles having asize of up to 300 nm on the surface were inspected and counted. Theresults are shown in Table 1 and SEM images are shown in FIGS. 1 to 6.Notably, the yttrium-base sprayed coating was formed by atmosphericplasma spraying using a gas mixture of 40 L/min of argon and 8 L/min ofhydrogen.

TABLE 1 Example Comparative Example 1 2 3 4 1 2 Spray material Y₂O₃Y₂O₃ + YF₃ YF₃ YOF Y₂O₃ YF₃ Coating thickness, 200 300 100 200 200 200μm Substrate material Al Al₂O₃ Si Al Al Al Cleaning Cleaning agenttartaric citric hydrofluoric lactic no no conditions acid acid acid +acid cleaning cleaning acidic ammonium fluoride Concentration, 2 10.05 + 0.1 2 — — mol/L Temperature, 30 50 30 50 — — ° C. Time, hr 4 120.5 12 — — Dissolution 2 20 2 10 — — depth, μm Particle population 0 0 00 numerous numerous on surface (particles/mm²) SEM image FIG. 1 FIG. 2FIG. 3 FIG. 4 FIG. 5 FIG. 6

As is evident from Table 1 and FIGS. 1 to 6, the yttrium-base sprayedcoatings in Examples 1 to 4 bear no particles on their surface whereasnumerous particles are on the yttrium-base sprayed coatings inComparative Examples 1 and 2 which omit cleaning with an aqueoussolution of acid or cleaning agent. It is readily presumed that theseparticles cause dust generation during etching treatment. When parts orcomponents having yttrium-base sprayed coatings of Examples 1 to 4deposited thereon are used, dusting as a result of yttrium-baseparticles spalling off during etching treatment in a semiconductordevice fabrication process is substantially prevented. This willeventually improve the fabrication yield of semiconductor devices.

Japanese Patent Application No. 2015-151568 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A method for preparing a yttrium-base sprayed coating, comprising thesteps of: thermally spraying a particulate spray material comprising atleast one compound selected from the group consisting of yttrium oxide,yttrium fluoride, and yttrium oxyfluoride to form a yttrium-base sprayedcoating having a thickness of 10 to 500 μm and chemically cleaning asurface of the coating with a cleaning liquid which is an organic acidaqueous solution, inorganic acid aqueous solution or organicacid/inorganic acid aqueous solution until a population of particleswith a size of up to 300 nm is no more than 5 particles per squaremillimeters of the coating surface.
 2. The method of claim 1 wherein thecleaning liquid is an aqueous solution of an acid selected from thegroup consisting of a monofunctional carboxylic acid, difunctionalcarboxylic acid, trifunctional carboxylic acid, hydroxy acid, sulfonicacid, nitric acid, sulfuric acid, carbonic acid, hydrofluoric acid, andacidic ammonium fluoride or a mixture thereof.
 3. The method of claim 2wherein in the cleaning liquid, the monofunctional carboxylic acid isformic acid or acetic acid, the difunctional carboxylic acid is maleicacid, tartaric acid or phthalic acid, the trifunctional carboxylic acidis citric acid, the hydroxy acid is lactic acid, and the sulfonic acidis methanesulfonic acid.
 4. The method of claim 1 wherein the chemicalcleaning step includes immersing the yttrium-base sprayed coating in thecleaning liquid to dissolve the coating to a depth of at least 0.01 μmfrom its surface for thereby removing particles with a size of up to 300nm on the coating surface.